Generally, a worm shaft and a worm wheel are used to transmit a driving force of a motor. When such a worm shaft and a worm wheel are made of a metal material, noise or vibration occurs at a portion contacting each other.
Accordingly, the worm shaft is made of a metal material such as steel, and the worm wheel is made of a synthetic resin material (or a plastic material) on a boss made of a metal such as steel in which the worm shaft is integrally formed with a gear portion having a toothed shape.
In the conventional method of manufacturing a worm wheel, a press-fitting method and an injection method are mainly used. In the press-fitting method, the boss cut to have a predetermined thickness is press-fitted into the center of the gear portion cut to have the same thickness as the boss and the bonding portion between the boss and the gear portion is fused at high frequency. Thereafter, teeth are post-processed to have a toothed shape on the outer circumferential surface of the gear portion to complete the manufacture of a worm wheel.
However, the conventional press-fitting method has a problem in that the manufacturing process is complicated and takes a long time to manufacture, and the post-processing is required to form the teeth, thereby lowering the productivity.
As a conventional prior art technology for solving the problem of the conventional press-fitting method, as disclosed in Korean Patent Laid-Open Publication No. 2009-0016130 (published on Feb. 13, 2009), there has been proposed a ‘method of manufacturing a worm wheel for a reducer gear of a steering apparatus using injection molding’, which manufactures a worm wheel by placing a boss in an injection mold and then discharging a resin melt to the injection mold.
However, the injection method of the worm wheel manufacturing method described above requires not only a hobbing process for post-processing additional teeth using a hob to the gear portion formed by injection but also faces a difficulty in a precise dimension control due to a resin shrinkage problem.
In addition, in the conventional injection method, the roundness and the concentricity of a hub and a boss coincide with each other due to an assembly error in which an axis is shaken during a process of press-fitting a worm wheel shaft into a worm wheel hub after molding a worm wheel boss and a gear forming portion which are individual elements of the worm wheel as a complete product, in which case, it is difficult to process the gear teeth using the hob.
As a prior art technology for improving this, as disclosed in Korean Patent Laid-Open Publication No. 10-2013-0100411 entitled “Worm wheel of an electric power assisted steering apparatus and method of manufacturing the same” (published on Sep. 11, 2013), the worm wheel is manufactured in a speed reducer of the electric power assisted steering apparatus, by integrally injection molding the hub, the boss, and the gear forming portion, to thereby improve formability of the worm wheel.
However, in this case, in view of the structural property formed at an inclined angle of the gear teeth in the gear forming portion, the gear forming portion is interfered with the injection mold due to the gear teeth during a process of taking out the gear forming portion from the injection mold. Accordingly, it is difficult to take out the gear forming portion from the injection mold and there is a possibility of occurrence of a deformation of the gear teeth interfered with the injection mold during the process of taking out the gear forming portion from the injection mold.